PM: domains: Restore comment indentation for generic_pm_domain.child_links

The rename of generic_pm_domain.slave_links to
generic_pm_domain.child_links accidentally dropped the TAB to align the
member's comment.  Re-add the lost TAB to restore indentation.

Fixes: 8d87ae48 ("PM: domains: Fix up terminology with parent/child")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
[ rjw: Minor subject edit ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>